1. Field of the Invention
The present invention relates to a plane light source unit for converting light of at least one point light sources efficiently into light of a plane light source to thereby make it possible to form a transmission type or reflection type liquid-crystal display device, which is excellent in luminance and uniformity of luminance, and which is bright and easy to view.
The present application is based on Japanese Patent Application No. Hei. 11-353141, which is incorporated herein by reference.
2. Description of the Related Art
A side-lighting type plane light source unit having a light source disposed on a side surface of a light pipe having prismatic structures was heretofore known as a plane light source unit capable of forming a transmission type or reflection type liquid-crystal display device using a back-lighting or front-lighting system. Generally, a cold-cathode tube was used as the light source in such a side-lighting type plane light source unit. There was, however, a problem that the cold-cathode tube needed high power consumption so that the frequency of required battery exchanges was high when the plane light source unit was used portably.
In consideration of the above problem, there was a proposal to arrange point light sources such as light-emitting diodes on an incidence side surface of a light pipe. Incidentally, the light-emitting diodes had advantages in reduction of power consumption, reduction of space and weight due to no use of any inverter, measures against electromagnetic wave, etc. From the point of view of prevention of disorder of a display image, improvement of light emission efficiency, etc., it was, however, difficult to incorporate a light scattering mechanism in the aforementioned light source unit. Hence, there was a problem that a large difference occurred between brightness and darkness to cause poor uniformity of luminance, because light was emitted like a stripe emission-line pattern corresponding to the location of arrangement of the point light sources. There was another proposal to arrange a light-emitting diode on a side surface of a rod-like light pipe to thereby form a linear light source. Also in this case, there was a problem that contrast of a display image ran short because a great deal of surface reflection occurred particularly when the linear light source was applied to a front-lighting system.
An object of the present invention is to develop a plane light source unit, which is capable of efficiently converting light of at least one point light source into light of a plane light source, which is excellent in luminance and uniformity of luminance, and which is capable of constituting a transmission type or reflection type liquid-crystal display device bright and easy to view.
According to the present invention, there is provided a plane light source unit comprising: a plane light pipe constituted by upper and lower surfaces and an incidence side surface which is one of side surfaces between the upper and lower surfaces; and a linear light pipe including a light supply surface and having a refractive index higher than that of the plane light pipe; wherein at least one point light source is disposed on the linear light pipe, and light incident from the at least one point light source is converted into light of a linear light source by the linear light pipe; and wherein the linear light pipe is disposed so as to make the light supply surface of the linear light pipe face the incidence side surface of the plane light pipe, and light of the linear light source exiting from the light supply surface so as to be incident on the incidence side surface is converted into light of a plane light source by means of the plane light pipe. There is further provided a liquid-crystal display device at least comprising the plane light source unit and a liquid crystal cell.
According to the present invention, the refractive index of a plane light pipe is decreased to suppress surface reflection while the refractive index of a linear light pipe is increased to attain improvement of output efficiency due to effective utilization of incident light. Hence, light of point light sources is converted, through the linear light pipe, into light of a linear light source efficiently. The converted light of the linear light source is converted again through the plane light pipe to thereby form a plane light source efficiently. Hence, there can be obtained a plane light source unit which is excellent in luminance and uniformity of luminance and which needs low power consumption. When the plane light source unit is used as a back-lighting or front-lighting system, there can be obtained a transmission type or reflection type liquid-crystal display device which needs low power consumption and which is bright, easy to view and excellent in display quality.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.